1. Field of the Invention
The present invention generally relates to a rocker arm of a center pivot type or an end pivot type adapted to be driven by a cam mounted on an internal combustion engine for selectively opening and closing a valve of a cylinder head and, more particularly, to the rocker arm prepared from a plate member such as a steel plate by the use of a press work.
2. Description of the Prior Art
A rocker arm utilized in a valve drive mechanism of an internal combustion engine has hitherto been manufactured by the use of a casting technique and, therefore, the manufacture of the rocker arm involves a number of problems to be solved such as those associated with considerable weight of the manufactured rocker arm, a relatively large number of manufacturing steps and a high cost of manufacture. In contrast thereto, the rocker arm of a plate type prepared from a shaped plate that is manufactured from a plate material of a relatively small thickness by the use of a press work has now come to be largely employed because of a light-weight feature, a relatively small number of manufacturing steps and a low cost of manufacture.
The rocker arm currently in use is generally available in two types; a center pivot type, in which the rocker arm is rockingly supported at a generally intermediate portion thereof, and an end pivot type in which the rocker arm is pivotally supported at one of its opposite ends through a pivot element secured to such one of the opposite ends. The rocker arm of the plate type is generally employed for both of the center pivot type and the end pivot type.
The center pivot type is of a design in which a valve driving member is adjustably fitted in a screw hole of an adjustment element formed in an arm body so that an adjustment can be made among a cam, a valve and a rocker arm. On the other hand, the end pivot type is of a design in which a pivot element is adjustably fitted in a screw hole of an adjustment element formed in an arm body so that an adjustment can be made among a cam, a valve and a rocker arm.
The center pivot type and the end pivot type differ from each other depending on which one of the valve driving member and the pivot element is used as an element to be mounted. However, both of those types employ the adjustment element and, accordingly, if the rocker arm is prepared by the use of a plate member of a relatively small thickness, a sufficient depth can hardly be obtained in the mounting hole for the adjustment element and a problem associated with securement of a sufficient strength around the mounting hole tends to arise.
By way of example, FIG. 30 illustrates the conventional center pivot type rocker arm such as disclosed in, for example, the Japanese Laid-open Patent Publication No. 2001-41011. The conventional rocker arm 51 shown therein includes an arm body 52 prepared from a single plate member by the use of a press work. The arm body 52 has a generally inverted U-shaped section defined by a pair of parallel side walls 53 and a connecting wall 54 bridging between respective upper edges of the side walls 53. One end of the connecting wall 54 that defines the adjustment element is formed with a female-threaded mounting hole 55 defined therein for adjustably receiving a valve driving member (not shown) therein. This mounting hole 55 is reinforced by formation of a cylindrical bulged portion 56 so that the valve driving member can be firmly retained. Also, when the rocker arm 51 is so designed that the respective centers of the adjustment element and a roller support structure is offset axially, the cylindrical bulged portion 56 will interfere with the side walls 53 unless any other measure is taken and, therefore, an open window 57 is defined in one of the side walls 53 on an offset side so as to avoid the interference.
While in the conventional rocker arm shown in FIG. 30 the cylindrical bulged portion 56 is formed in the mounting hole 55 in the connecting wall 54 to secure a required depth of the mounting hole 55, formation of the mounting hole 55 is complicated and time-consuming because of the cylindrical bulged portion 56 so formed. Where the offset structure is employed, the open window 57 is defined to avoid interference with the cylindrical bulged portion 56, but the amount of offset is limited only with a play afforded by the open window 57.
Also, in this conventional rocker arm, if the arm body 52 of a relatively large thickness is employed, a sufficient strength can be secured around pivot support holes 58 defined in the respective side walls 53. However, since the plate thickness of the entire arm body 52 of the generally inverted U-shaped section increases, the connecting wall 54 which does not require a relatively high strength will have a correspondingly increased thickness, resulting in inconvenience in reducing the overall weight of the arm body. In addition, the amount of material used to manufacture the arm body will unnecessarily increase.
FIG. 31 illustrates the conventional rocker arm of the end pivot type such as disclosed in, for example, the Japanese Laid-open Patent Publication No. 2001-132414. This conventional rocker arm 81 includes an arm body 82 of a generally inverted U-shaped section having one end thereof provided with an end overlap piece 83 extending in a direction lengthwise of the arm body 82, which overlap piece 83 is turned backwardly to define an adjustment element of an overlapped structure. A mounting hole 85 is defined in this adjustment element of the overlapped structure, in which a pivot member (not shown) is adjustably inserted. Since in this example, the adjustment element is of the overlapped structure, a sufficient strength can be secured around the mounting hole 85.
However, since the end overlap piece 83 has to be turned backwardly and a step of turning the end overlap piece 83 backwardly, which is separate from a step of shaping the arm body 82 so as to have a generally inverted U-shaped section, is required, the number of manufacturing steps consequently increases, resulting in increase of the manufacturing cost.
Also, in order for the mounting hole 85 to be formed after the end overlap piece 83 has been turned backwardly, the distance L from the mounting hole 85 to a bent portion at which the end overlap piece 83 is turned backwardly as shown in FIG. 31 has to be somewhat large. In other words, since a bending process is employed to turn the end overlap piece 83 backwardly, degradation of material occurs in a portion of the end overlap piece 83 and/or the connecting wall 84 adjacent the bent portion and, therefore, they are not suited to formation of the mounting hole 85. Particularly where the mounting hole 85 is employed in the form of an internally threaded screw hole, inconveniences will undoubtedly arise. For this reason, it is necessary to provide a sufficient margin which enables the arm body to overcome detrimental effects possibly brought about by the degradation of material. Since as discussed above the front end portion of the arm body 82 is required to have an increased length inclusive of the above mentioned margin, the rocker arm 81 as a whole has a correspondingly increased length, far from being manufactured compact. Even where the adjustment element of the overlapped structure defined by the end overlap piece 83 is applied to the center pivot type shown in and described with reference to FIG. 30, a similar problem associated with the lengthwise dimension as discussed above will arise.
FIG. 32 illustrates the center pivot type rocker arm suggested in, for example, the Japanese Laid-open Patent Publication No. 10-299430. A rocker arm 61 shown therein is of a generally folded structure formed by bending an elongated plate member about a substantially intermediate portion 61b thereof so as to define two side walls 63 and 64, with the bent portion 61b defining an adjustment element in which a valve driving member (not shown) is adjustably inserted.
In this conventional rocker arm of the center pivot type shown in FIG. 32, a mounting hole 75 defined in the adjustment element can be formed to have a relatively great depth, but since the arm body is defined only by the mutually confronting side walls 63 and 64, means is required to regulate the space between the side walls 63 and 64 to a predetermined or required value. It is to be noted that the side walls 63 and 64 are formed with respective cylindrical bulged portions 70 positioned around associated pivot support holes 69 and, accordingly, a sufficient strength can be secured even though the rocker arm 61 is prepared from a thin plate material. However, formation of the cylindrical bulged portions 70 is indeed time-consuming and laborious.
Also, since in the conventional rocker arm 61 shown in FIG. 32, there is no means by which the space between the side walls 63 and 64 is regulated to a predetermined or required value, an extra space regulating means is required.
While in describing the prior art believed to be pertinent to the present invention reference has been made to the center pivot type, similar problems to those discussed above equally applies even to the rocker arm of the end pivot type.